I need the title of a book?

i read a book a couple years back that i would like to re-read but i cannot for the life of me remember its title. all i know is that it had to do with a dystopian society that was divided into sections or areas of some sort, and some of the areas were off limits. the main character meets a mentor in one of these off limits sections and the mentor makes him separate salt from sugar by hand. also, i think the mentor ends up being the society's ruler's son. anyone know what book i am talking about?